1. Technical Field
The present disclosure relates to semiconductor devices, and more particularly CMOS image sensors and to methods of forming the same.
2. Description of the Related Art
An image sensor is a device that typically converts an optical image into an electrical signal. Image sensors are generally categorized as either complementary metal-oxide-silicon (CMOS) image sensors or charge coupled device (CCD) image sensors. Moreover, there are certain benefits in using CMOS as opposed to CCD image sensors and vice versa. For example, CCD image sensors may have beneficial properties with respect to photosensitivity and noise, in comparison to the CMOS image sensors. However, CCD image sensors may have difficulties with respect to high integration and have high power consumption. On the other hand, CMOS image sensors have beneficial properties with respect to high integration and have low power consumption, and CMOS image sensors may also be manufactured relatively simply. The technology of conventional CMOS image sensors will be described below.
For example, a pixel of a CMOS image sensor typically includes photodiodes which receive light and CMOS elements which control image signals input from the photodiodes. Moreover, photodiodes typically include two doped regions doped with impurities of opposite conductivity types. Each of the doped regions included in the photodiodes has the same range of projection (Rp) irrespective of the wavelength of the incident light. An electron-hole pair is generated in the photodiodes, depending upon the wavelength and the intensity of red, green, and blue lights received through a color filter, and an output signal is changed according to the amount of the electrons generated, thereby sensing an image. Also, the conventional CMOS image sensor includes a deep well region under the photodiodes to control a depletion layer region of the photodiodes and thus prevent a leakage current. Additionally, the general deep well region is typically uniformly distributed throughout a pixel region of the conventional CMOS image sensor.
Also, in a conventional CMOS image sensor, the more the wavelength of light increases, the more the penetration depth of the light increases, but the absorption coefficient in a silicon layer also decreases. Consequently, as a result of the above, electron-hole pairs, generated by red and green lights with a long wavelength, may be generated adjacently to the deep well under the photodiode and lights with a long wavelength may also not enter perpendicularly to the CMOS image sensor. Also, electron-hole pairs may be generated under a device isolation layer or a CMOS element (e.g., transistor). Furthermore, the electrons generated adjacently to the deep well or under the device isolation layer or under the transistor may not enter into a photodiode of a corresponding pixel, but rather may instead be diffused into a photodiode of an adjacent pixel. Accordingly, as a result, the sensitivity of the CMOS image sensor may be decreased and cross-talk may occur.
Thus, there is a need for a CMOS image sensor having improved photosensitivity and which is capable of preventing cross-talk and to a method of forming the same.